add changelog edits to release plan

[romainmenke]

Release plans will now also add entries in changelogs of downstream packages.

example commit : romainmenke/postcss-mono-repo-playground@b4fb9d2

This should streamline the process of releasing complex updates.

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After a single chunk of releases the git repo should now always have clean git state.
It might be best to have a process like this :

1. npm ci
2. release-plan:dry-run to verify what will be released in the current chunk
3. release-plan to release stuff
4. push to main
5. verify the last commit with the updated versions and changelogs
6. go to 2. and repeat

[Antonio-Laguna]

LGTM! Thanks for tackling this, can't wait to have a big chunk and see how it behaves!